Properties Of Neutron Stars Research Articles

Effects of free parameterized TOV equations on properties of neutron stars with hyperons

In view of the crucial role of the structure in neutron star (NS) research, parameterized TOV (PTOV) equations are used to calculate the static NS sequence with five parameters including [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text] and [Formula: see text], which represent the five factors that impact the NS structure. In order to promote the efficiency in the combination of astronomical observations and the theory, we investigate the meticulous influences of the five factors on NS structure with various hyperons in this work. Results show that the maximum masses of NSs are reduced with the free parameters, respectively, whereas the variation trend of corresponding radii is wavy in some cases. Especially, when hyperons are included, the macroscopic properties of NSs, except redshift, are relatively smaller than those in conventional NS matter. The effects on conventional NSs and hyperonic NSs are quite different in the high NS mass area but diminish when the stellar mass is lower than approximately 1.5 times solar mass. Finally, comparing with observations, we predict constraints on the free parameters.

Linear to circular conversion in the polarized radio emission of a magnetar

AbstractRadio emission from magnetars provides a unique probe of the relativistic, magnetized plasma within the near-field environment of these ultra-magnetic neutron stars. The transmitted waves can undergo birefringent and dispersive propagation effects that result in frequency-dependent conversions of linear to circularly polarized radiation and vice versa, thus necessitating classification when relating the measured polarization to the intrinsic properties of neutron star and fast radio burst emission sites. We report the detection of such behaviour in 0.7–4 GHz observations of the P = 5.54 s radio magnetar XTE J1810−197 following its 2018 outburst. The phenomenon is restricted to a narrow range of pulse phase centred around the magnetic meridian. Its temporal evolution is closely coupled to large-scale variations in magnetic topology that originate from either plastic motion of an active region on the magnetar surface or free precession of the neutron star crust. Our model of the effect deviates from simple theoretical expectations for radio waves propagating through a magnetized plasma. Birefringent self-coupling between the transmitted wave modes, line-of-sight variations in the magnetic field direction and differences in particle charge or energy distributions above the magnetic pole are explored as possible explanations. We discuss potential links between the immediate magneto-ionic environments of magnetars and those of fast radio burst progenitors.

Impact of modified gravity theory on neutron star and nuclear matter properties

Impact of modified gravity theory on neutron star and nuclear matter properties

Insights into Neutron Star Equation of State by Machine Learning

Due to its powerful capability and high efficiency in big data analysis, machine learning has been applied in various fields. We construct a neural network platform to constrain the behaviors of the equation of state of nuclear matter with respect to the properties of nuclear matter at saturation density and the properties of neutron stars. It is found that the neural network is able to give reasonable predictions of parameter space and provide new hints into the constraints of hadron interactions. As a specific example, we take the relativistic mean field approximation in a widely accepted Walecka-type model to illustrate the feasibility and efficiency of the platform. The results show that the neural network can indeed estimate the parameters of the model at a certain precision such that both the properties of nuclear matter around saturation density and global properties of neutron stars can be saturated. The optimization of the present modularly designed neural network and extension to other effective models is straightforward.

Symmetry energy and neutron star properties constrained by chiral effective field theory calculations

Symmetry energy and neutron star properties constrained by chiral effective field theory calculations

The impact of anisotropy on neutron star properties: insights from 𝖨–𝖿–𝖢 universal relations

Anisotropy in pressure within a star emerges from exotic internal processes. In this study, we incorporate pressure anisotropy using the Quasi-Local model. Macroscopic properties, including mass (M), radius (R), compactness (C), dimensionless tidal deformability (Λ), the moment of inertia (I), and oscillation frequency (f), are explored for the anisotropic neutron star. Magnitudes of these properties are notably influenced by anisotropy degree. Universal I–f–C relations for anisotropic stars are explored in this study. The analysis encompasses various EOS types, spanning from relativistic to non-relativistic regimes. Results show the relation becomes robust for positive anisotropy, weakening with negative anisotropy. The distribution of f-mode across M–R parameter space as obtained with the help of C–f relation was analyzed for different anisotropic cases. Using tidal deformability data from GW170817 and GW190814 events, a theoretical limit for canonical f-mode frequency is established for isotropic and anisotropic neutron stars. For isotropic case, canonical f-mode frequency for GW170817 event is f 1.4 = 2.606+0.457 -0.484kHz; for GW190814 event, it is f 1.4 = 2.097+0.124 -0.149kHz. These relationships can serve as reliable tools for constraining nuclear matter EOS when relevant observables are measured.

Exploring robust correlations between fermionic dark matter model parameters and neutron star properties: A two-fluid perspective

Exploring robust correlations between fermionic dark matter model parameters and neutron star properties: A two-fluid perspective

Nuclear Matter Properties and Neutron Star Phenomenology Using the Finite Range Simple Effective Interaction

The saturation properties of symmetric and asymmetric nuclear matter have been computed using the finite range simple effective interaction with Yukawa form factor. The results of higher-order derivatives of the energy per particle and the symmetry energy computed at saturation, namely, Q0, Ksym, Kτ, Qsym, are compared with the corresponding values extracted from studies involving theory, experiment and astrophysical observations. The overall uncertainty in the values of these quantities, which results from a wide spectrum of studies described in earlier literature, lies in the ranges −1200≲Q0≲400 MeV, −400≲Ksym≲100 MeV, −840≲Kτ≲−126 MeV and −200≲Qsym≲800 MeV, respectively. The ability of the equations of state computed with this simple effective interaction in predicting the threshold mass for prompt collapse in binary neutron star merger and gravitational redshift has been examined in terms of the compactness of the neutron star and the incompressibility at the central density of the maximum mass star. The correlations existing between neutron star properties and the nuclear matter saturation properties have been analyzed and compared with the predictions of other model calculations.

Spin Evolution of Neutron Stars

In this paper we review the basics of magneto-rotational properties of neutron stars focusing on spin-up/spin-down behavior at different evolutionary stages. The main goal is to provide equations for the spin frequency changes in various regimes (radio pulsar, propeller, accretor, etc.). Since presently the spin behavior of neutron stars at all stages remains a subject of many uncertainties, we review different suggestions made over the years in the literature.

On the Maximum Mass and Oblateness of Rotating Neutron Stars with Generic Equations of State

A considerable effort has been dedicated recently to the construction of generic equations of state (EOSs) for matter in neutron stars. The advantage of these approaches is that they can provide model-independent information on the interior structure and global properties of neutron stars. Making use of more than 106 generic EOSs, we assess the validity of quasi-universal relations of neutron-star properties for a broad range of rotation rates, from slow rotation up to the mass-shedding limit. In this way, we are able to determine with unprecedented accuracy the quasi-universal maximum-mass ratio between rotating and nonrotating stars and reveal the existence of a new relation for the surface oblateness, i.e., the ratio between the polar and equatorial proper radii. We discuss the impact that our findings have on the imminent detection of new binary neutron-star mergers and how they can be used to set new and more stringent limits on the maximum mass of nonrotating neutron stars, as well as to improve the modeling of the X-ray emission from the surface of rotating stars.

Hyperonic uncertainties in neutron stars, mergers and supernovae

Abstract In this work we delve into the temperature-dependent Equation of State (EoS) of baryonic matter within the framework of the FSU2H* hadronic model, which comprehensively incorporates hyperons and is suitable for relativistic simulations of neutron star mergers and supernovae. To assess the impact of the uncertainties in the hyperonic sector on astrophysical observables, we introduce two additional models, namely FSU2H*L and FSU2H*U. These models cover the entire spectrum of variability of hyperonic potentials, as derived from experimental data. Our investigations reveal that these uncertainties extend their influence not only to the relative abundances of various particle species but also to the EoS itself and, consequently, have an impact on the global properties of both cold and hot neutron stars. Notably, their effects become more pronounced at large temperatures, owing to the increased presence of hyperons. These findings have direct implications for the outcomes of relativistic simulations of neutron star mergers and supernovae, emphasizing the need of accounting for hyperonic uncertainties to ensure the accuracy and reliability of such simulations in astrophysical contexts.

Towards Uncovering Dark Matter Effects on Neutron Star Properties: A Machine Learning Approach

Over the last few years, researchers have become increasingly interested in understanding how dark matter affects neutron stars, helping them to better understand complex astrophysical phenomena. In this paper, we delve deeper into this problem by using advanced machine learning techniques to find potential connections between dark matter and various neutron star characteristics. We employ Random Forest classifiers to analyze neutron star (NS) properties and investigate whether these stars exhibit characteristics indicative of dark matter admixture. Our dataset includes 32,000 sequences of simulated NS properties, each described by mass, radius, and tidal deformability, inferred using recent observations and theoretical models. We explore a two-fluid model for the NS, incorporating separate equations of state for nucleonic and dark matter, with the latter considering a fermionic dark matter scenario. Our classifiers are trained and validated in a variety of feature sets, including the tidal deformability for various masses. The performance of these classifiers is rigorously assessed using confusion matrices, which reveal that NS with admixed dark matter can be identified with approximately 17% probability of misclassification as nuclear matter NS. In particular, we find that additional tidal deformability data do not significantly improve the precision of our predictions. This article also delves into the potential of specific NS properties as indicators of the presence of dark matter. Radius measurements, especially at extreme mass values, emerge as particularly promising features. The insights gained from our study are pivotal for guiding future observational strategies and enhancing the detection capabilities of dark matter in NS. This study is the first to show that the radii of neutron stars at 1.4 and 2.07 solar masses, measured using NICER data from pulsars PSR J0030+0451 and PSR J0740+6620, strongly suggest that the presence of dark matter in a neutron star is more likely than only hadronic composition.

Influence of hyperon–hyperon interaction on the properties of neutron stars

The properties of neutron stars are studied in a composite model of the strong interaction. In the regime of low to medium baryonic densities a covariant hadronic model is adopted which includes an exclusive channel for the hyperon–hyperon interaction mediated by hidden strangeness mesons, which in turn couple to other mesons through polynomial vertices. The new coupling constants are subject to phenomenological constraints. The presence of free quarks in the core of the star is considered by using the Nambu–Jona Lasinio model supplemented with a vector interaction. The deconfinement process is described by a continuous coexistence of phases. Several structure parameters of neutron stars, such as mass-radius relation, moment of inertia, tidal deformability, and the propagation of nonradial f and g-modes within the relativistic Cowling approximation are studied. The predictions of the model are in good agreement with recent observational data, in particular the maximum inertial mass is greater than the observational lower limit of two solar masses.

Isovector properties of finite nuclei: Constraints from neutron stars observations

The nuclear symmetry energy plays a crucial role in the structure of finite nuclei and the bulk properties of neutron stars. However, its values at high densities are highly uncertain, and the corresponding experimental data have large errors. One way to determine or at least estimate these high-density values is through neutron star observations. Recently, observations of gravitational waves from binary neutron star mergers have provided useful information on their radius and tidal deformability, which are directly related to the symmetry energy. This work attempts to use recent observations to constrain the structure of finite nuclei. Specifically, we parameterize the equation of state (EoS) describing asymmetric and symmetric nuclear matter using the parameter η = (K0L2)1/3, where K0 is the incompressibility and L is the slope parameter, The parameter η regulates the stiffness of the EoS, and we expect its values to affect both finite nuclei and neutron star properties, especially given the important role of isovector interactions. It is natural to expect that constraints on η for finite nuclei will also imply constraints on neutron star properties and vice versa. In light of the above, we propose a simple yet self-consistent method to simultaneously examine the effects of η on the properties of finite nuclei and neutron stars. We found constraints on these systems by combining recent experimental data (PREX-2) and observational data from the LIGO and Virgo detectors.

Quarkyonic matter and applications in neutron stars

The structure and basic properties of dense nuclear matter still remain one of the open problems of physics. In particular, the composition of the matter that composes neutron stars is under theoretical and experimental investigation. Among the theories that have been proposed, apart from the classical one where the composition is dominated by hadrons, the existence or coexistence of free quark matter is a dominant guess. An approach towards this solution is the phenomenological view according to which the existence of quarkyonic matter plays a dominant role in the construction of the equation of state (EOS). In this paper we propose a phenomenological model for quarkyonic matter, borrowed from corresponding applications in hadronic models, where the interaction in the quarkyonic matter depends not only on the position but also on the momentum of the quarkynions. This consideration, as we demonstrate, can have a dramatic consequence on the shape of the EOS and thus on the properties of neutron stars.

Sound velocity, equation of state and strangeness in neutron star matter

This presentation in two parts starts with a brief review on the speed of sound in neutron star matter as inferred from observational data. It is pointed out that, in view of the strong constraints imposed by observed properties of the heaviest neutron stars, the equation of state must be very stiff. Stringent limits can be set on the possible appearance of phase transitions in neutron star cores. The second part discusses issues concerning the occurance of strangeness degrees of freedom, in particular of hyperons, in neutron stars. Here the focus is on the role of repulsive hyperon-nuclear three-body forces, potentially resolving the so-called hyperon puzzle.

HESS J1731-347 and the existence of exotic matter: Kaon condensation in neutron stars

The recent observation of a compact star with the lowest observable mass of 0.77+0.20 –0.17 M⊙ within the supernova remnant HESS J1731-347 has redefined our understanding for the dense nuclear matter equation of state and enhances the existence of exotic matter. In the present work, we investigate the possible existence of kaon condensation in hadronic neutron stars through the framework of the Momentum-Dependent Interaction nuclear model describing the dense nuclear matter. We concentrate on the strangeness content of the proton and its implications on both the microscopic and macroscopic properties of neutron stars. The aforementioned quantity is of interest as it is also directly related to the location of the phase transition to kaons. The simultaneous description of the compact object within the HESS J1731-347 and the maximum observable neutron star mass imposes severe constraints on the strangeness content of the proton and the critical density for kaon condensation. The present study along with observations of neutron stars, may help to provide tighter constraints on the equation of state of dense nuclear matter and, even more, to shed light to the existence of an exotic core in neutron stars.

Measurements of p − Λ and d − Λ correlations in 3 GeV Au+Au collisions at STAR

Heavy-ion collisions provide a unique opportunity to explore nucleon-hyperon (N-Y) interactions through two-particle correlations. The p − Λ and d − Λ correlations shed light on both N-Y two-body and N-N-Y three-body interactions, which is crucial for understanding neutron star properties. We present the high precision measurement of p − Λ and the first measurement of d − Λ correlation with √SNN = 3 GeV Au+Au collisions at STAR. Using the Lednicky-Lyuboshitz formalism, we characterized emission source size, the scattering length (f0), and the effective range (d0) of p − Λ and d − Λ interactions. Using the f0 and d0 extracted from two spin states in d − Λ correlation, the parameters from the doublet state indicate the hypertriton binding energy is consistent with the current average of world measurements.

Tidal properties of neutron stars in scalar-tensor theories of gravity

Tidal properties of neutron stars in scalar-tensor theories of gravity

Establishing connection between neutron star properties and nuclear matter parameters through a comprehensive multivariate analysis

Establishing connection between neutron star properties and nuclear matter parameters through a comprehensive multivariate analysis